Date: Sat, 4 Aug 2018 15:38:56 -0400
From: Markus Vester <markusvester@aol.com>
To: rsgb_lf_group@blacksheep.org
Message-Id: <16506732edf-c8c-186c@webjasstg-vab24.srv.aolmail.net>
In-Reply-To: <5B61E6FB.7090507@posteo.de>
MIME-Version: 1.0
Subject: Re: VLF:Ground loop antennas, how to tune best?
Content-Type: multipart/alternative; 
	boundary="----=_Part_7452_506515056.1533411536607"
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

------=_Part_7452_506515056.1533411536607
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Hi Stefan,

sorry maybe a bit late...

Re lab experiment:
Although the insertion of the LC series resonant circuit shouldn't make a d=
ifference at the fundamental frequency, it will create a high inductive loa=
d impedance at high harmonics. Presumably the 3 mH resonates with the FET o=
utput capacitance at 10 x f, leading to excessive gain for that harmonic, o=
r potentially even approaching self oscillation.

Re inductance compensation by shunt capacitor:
In my opinion this is the preferable method. Due to the low Q-factor involv=
ed, I probably wouldn't even call it "parallel resonance" but rather low-pa=
ss compensation. Besides avoiding the impedance pole, it is also "broadband=
" and may work reasonably well from DC to 8 kHz with a single fixed capacit=
ance.=20

At DL0AO, I've set up a 424 uHz instance at 2.47 kHz - perhaps a bit optimi=
stic but who knows.

Best 73,
Markus (DF6NM)

-----Urspr=C3=BCngliche Mitteilung-----=20
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mi, 1. Aug 2018 19:01
Betreff: VLF:Ground loop antennas, how to tune best?


Hi all,

Today i played a bit with my linear VLF/ULF PA, some output=20
transformers, a 50 Ohm dummy load and a 3 mH air cored coil (a smal one,=20
from LF). I thought this could represent the ground loop antenna during=20
tests in the shack.

With the switchable C-network, i managed to tune to resonance between=20
8.27 kHz down to 0.97 kHz. A usual R-L-C network. The goal was to=20
proove that the system can tune to and run 1 A antenna current on each=20
of the desired frequencies.

However, i did not manage to reach more than 500 mA 'antenna' current.=20
Above, there appeared significant distortions / harmonics. Why?
When i connect the dummy load directly to the output transformer (just R=20
instead of RLC), i can easily tune to 1 A 'antenna' current and the sine=20
wave almost looks perfectly. So it cannot be an issue of a saturating=20
transformer. Also the coil cannot saturate.
So, where are the distortions coming from?
The main component of the overlayed distortion voltage is maybe 10x=20
higher in frequency.

BTW in the first test with the 900m ground loop the voltage showed some=20
distortions as well and i wondered how it comes from. Then i thought it=20
has to do with saturation and a far-from-ideal matching of the PA output=20
impedance...

Back to the test in the shack:
Eventually there are further resonance frequencies, coming from the=20
stray inductance of the transformer and the 'internal' capacity of the coil=
.

Now if this can happen with discrete elements in the shack, it can=20
happen on the ground loop anyway! This antenna will certainly have an=20
interesting frequency response, TX-wise.

So my thought is: Maybe it is even better not to series resonate the=20
loop but to parallel resonate it! This will lead to a higher feed point=20
impedance, which will be frequency dependent, so it is a more complex=20
thing. But the parallel C should help to avoid transmitting on harmonics.

Or, i could series resonate the antenna on the frequency of interest and=20
then add a parallel resonated LC circuit. This is easier to do because=20
the parallel LC can be tuned before connecting the antenna. Then the=20
antenna can be connected...
Or, even something like a T-filter (2 series L and one parallel C) but=20
that will be complex to align when sitting in the car which is full of=20
electronis stuff anyway... Also it is a challenge on ULF :-)

And certainly it helps to minimise the stray reactances, e.g. by using=20
just as less primary turns on the output transformer as possible. But=20
then it is not usable on all frequencies and will need taps on the=20
primary and secondary side.

Any ideas / comments? :-)

73, Stefan
------=_Part_7452_506515056.1533411536607
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

<div style=3D"font-size:10pt;font-family:arial,helvetica,sans-serif;color:b=
lack;"><div style=3D"color: black; font-family: arial,helvetica,sans-serif;=
 font-size: 10pt;">Hi Stefan,<br><br>sorry maybe a bit&nbsp;late...<br><br>=
Re lab experiment:<br>Although&nbsp;the insertion&nbsp;of the LC series res=
onant circuit shouldn't make a difference at the fundamental frequency, it =
will create a high&nbsp;inductive load impedance at high harmonics. Presuma=
bly the 3 mH resonates with the FET output capacitance at 10 x f,&nbsp;lead=
ing to excessive gain&nbsp;for that harmonic, or&nbsp;potentially even appr=
oaching self oscillation.<br><br>Re inductance compensation by shunt capaci=
tor:<br>In my opinion this is the preferable method. Due to&nbsp;the low Q-=
factor involved, I probably wouldn't even call it "parallel resonance" but =
rather low-pass compensation. Besides&nbsp;avoiding the impedance pole, it =
is&nbsp;also "broadband"&nbsp;and may work reasonably well from DC to 8 kHz=
 with a single fixed&nbsp;capacitance.&nbsp;<br><br>At DL0AO, I've&nbsp;set=
 up a&nbsp;424 uHz instance at 2.47 kHz - perhaps a bit optimistic but who =
knows.<br><br>Best 73,<br>Markus (DF6NM)<br><br>-----Urspr=C3=BCngliche Mit=
teilung----- <br>Von: DK7FC &lt;selberdenken@posteo.de&gt;<br>An: rsgb_lf_g=
roup &lt;rsgb_lf_group@blacksheep.org&gt;<br>Verschickt: Mi, 1. Aug 2018 19=
:01<br>Betreff: VLF:Ground loop antennas, how to tune best?<br><br></div>Hi=
 all,<br><br>Today i played a bit with my linear VLF/ULF PA, some output <b=
r>transformers, a 50 Ohm dummy load and a 3 mH air cored coil (a smal one, =
<br>from LF). I thought this could represent the ground loop antenna during=
 <br>tests in the shack.<br><br>With the switchable C-network, i managed to=
 tune to resonance between <br>8.27 kHz down to 0.97 kHz. A usual R-L-C net=
work. The goal  was to <br>proove that the system can tune to and run 1 A a=
ntenna current on each <br>of the desired frequencies.<br><br>However, i di=
d not manage to reach more than 500 mA 'antenna' current. <br>Above, there =
appeared significant distortions / harmonics. Why?<br>When i connect the du=
mmy load directly to the output transformer (just R <br>instead of RLC), i =
can easily tune to 1 A 'antenna' current and the sine <br>wave almost looks=
 perfectly. So it cannot be an issue of a saturating <br>transformer. Also =
the coil cannot saturate.<br>So, where are the distortions coming from?<br>=
The main component of the overlayed distortion voltage is maybe 10x <br>hig=
her in frequency.<br><br>BTW in the first test with the 900m ground loop th=
e voltage showed some <br>distortions as well and i wondered how it comes f=
rom. Then i thought it <br>has to do with saturation and a far-from-ideal m=
atching of the PA output <br>impedance...<br><br>Back to the test in the sh=
ack:<br>Eventually there are further resonance frequencies, coming from the=
 <br>stray inductance of the transformer and the 'internal' capacity of the=
 coil.<br><br>Now if this can happen with discrete elements in the shack, i=
t can <br>happen on the ground loop anyway! This antenna will certainly hav=
e an <br>interesting frequency response, TX-wise.<br><br>So my thought is: =
Maybe it is even better not to series resonate the <br>loop but to parallel=
 resonate it! This will lead to a higher feed point <br>impedance, which wi=
ll be frequency dependent, so it is a more complex <br>thing. But the paral=
lel C should help to avoid transmitting on harmonics.<br><br>Or, i could se=
ries resonate the antenna on the frequency of interest and <br>then add a p=
arallel resonated LC circuit. This is easier to do because <br>the parallel=
 LC can be tuned before connecting the antenna. Then the <br>antenna can be=
 connected...<br>Or, even something like a T-filter (2 series L and one par=
allel C) but <br>that will be complex to align when sitting in the car whic=
h is full of <br>electronis stuff anyway... Also it is a challenge on ULF :=
-)<br><br>And certainly it helps to minimise the stray reactances, e.g. by =
using <br>just as less primary turns on the output transformer as possible.=
 But <br>then it is not usable on all frequencies and will need taps on the=
 <br>primary and secondary side.<br><br>Any ideas / comments? :-)<br><br>73=
, Stefan<br><br></div>
------=_Part_7452_506515056.1533411536607--